Multicolor, single active layer electrochromic devices

ABSTRACT

An electrochromic device including a single unitary active layer with a dye having a nitrogen group and a conducting polymer having a nitrogen group. The active layer has a first color in an oxidized state and a second color in a reduced state; the electrochromic device has no other active layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation from the U.S. patent application Ser.No. 16/161,667 filed on Oct. 16, 2018, which claims priority from theU.S. Provisional Patent Application No. 62/572,775 filed Oct. 16, 2017.The disclosure of each of the above-identified patent applications isincorporated herein by reference.

RELATED ART

Electrochromic devices are devices whose optical properties, such aslight transmission and absorption, can be altered in a reversible mannerthrough the application of a voltage. This property enableselectrochromic devices to be used in various applications, such as smartwindows, electrochromic mirrors, and electrochromic display devices.

Most commercially available electrochromic devices are relativelycomplex devices that comprise multiple layers (e.g., 3-5 layers) ofdifferent materials that are required for the device to change state. Inaddition to their complexity, such devices can require expensiveprocesses, materials, or equipment to manufacture. Furthermore, suchdevices typically can only be placed in a light-transmitting state or alight-blocking state and cannot be placed in alternative colored states.In view of this, it would be desirable to have relatively simpleelectrochromic devices that can be placed in multiple colored states.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

The present disclosure may be better understood with reference to thefollowing figures. Matching reference numerals designate correspondingparts throughout the figures, which are not necessarily drawn to scale.

FIG. 1 is a partial side view of an embodiment of a single active layerelectrochromic device.

FIGS. 2A and 2B together present a table that identifies chemicalstructures of various dyes and includes images of the oxidized andreduced states of electrochromic devices that comprise an active layerincorporating a dye.

FIGS. 3A, 3B, 3C, 3D, 3E, 3F, 3G, and 3H are scanning electronmicroscopy (SEM) images of various examples of active layers.

FIG. 4 is a graph that shows the UV-visible absorption spectra of activelayers containing various example dyes.

FIGS. 5A, 5B, and 5C are graphs that show the change of UV-visabsorption at potentials 1.7 and −0.5 V for various example activelayers.

FIGS. 6A, 6B, 6C, 6D, 6E, and 6F are graphs that show the cyclicvoltammetry (CV) of various example active layers containingPVA+APS+PANI gel.

FIGS. 7A, 7B, 7C, 7D, 7E, and 7F are graphs that show the results ofchrono-amperometric studies of various example active layers.

FIG. 8 is a schematic that illustrates an example mechanism forcoloration and decoloration of PVA+APS+MO+PANI based active layers.

FIG. 9 is a schematic that illustrates an example mechanism forcoloration and decoloration of PVA+APS+CR+PANI based active layers.

FIG. 10 is a schematic that illustrates an example mechanism forcoloration and decoloration of PVA+APS+RB+PANI based active layers.

DETAILED DESCRIPTION

As described above, it would be desirable to have relatively simpleelectrochromic devices that can be placed in multiple colored states.Disclosed herein are examples of such devices. In some embodiments, theelectrochromic devices comprise a single active layer that can betransitioned from a first colored state to a second colored state bycontrolling the electrical potential applied across the active layer. Insome embodiments, the active layer comprises a dye and a conductingpolymer that both comprise a nitrogen group. When a positive potentialis applied to the active layer, the dye and the conducting polymer areeach oxidized and each adopts a relatively dark color. These colorscombine to create a relatively dark combination color (i.e., the firstcolored state). When a negative (reversing) potential is applied to theactive layer such that the active layer has little or no electricalpotential, the dye and the conducting polymer are reduced. As a resultof this reduction, the dye adopts a relatively light color, while theconducting polymer becomes completely or substantially transparent. As aresult, the combination color in the reduced state (i.e., the secondcolored state) is primarily the color of the dye in its reduced state.

In the following disclosure, various specific embodiments are described.It is to be understood that those embodiments are exampleimplementations of the disclosed inventions and that alternativeembodiments are possible. All such embodiments are intended to fallwithin the scope of this disclosure. Example electrochromic devices inaccordance with this disclosure will now be discussed. It is noted thatthe electrochromic devices, can be used alone or can be incorporatedinto other objects. For example, the disclosed electrochromic devicescan be used as or incorporated into display devices to change the colorof the display devices. In other cases, the disclosed electrochromicdevices can be used as or incorporated into windows.

The electrochromic devices generally comprise a single, unitary activelayer of material, or “active layer,” that is positioned between firstand second transparent or translucent inactive layers of material, or“layers.” As used herein, the term “active layer” refers to a layer ofmaterial that is configured to change color when a potential is appliedto the active layer. When it is expressed that a “single” active layeris provided in the electrochromic device, this means that there is onlyone layer in the device that contains components that contribute to thecolor change reaction. Such an active layer can be contrasted with an“inactive layer,” which is a layer that serves a different purposewithin the device, such as simply delivering electricity to the activelayer. In some embodiments, the inactive layers comprise thintransparent glass or plastic plates that are coated with a transparent,electrically conductive film. The transparent, electrically conductivefilms can, in some embodiments, comprise a transparent conducting oxide(TCO), such as indium tin oxide (ITO), fluorine doped tin oxide (FTO),or doped zinc oxide (ZnO). Irrespective of the composition of thetransparent, electrically conductive films, the films at least cover thesurfaces of the layers that face the active layer. FIG. 1 illustrates anexample of a single active layer electrochromic device 10. As shown inthis figure, the device 10 comprises an active layer 12 that issandwiched between two electrically conductive inactive layers 14.Accordingly, in the example of FIG. 1 , the device 10 requires onlythree layers and is, therefore, very simple in construction.

The active layer comprises multiple components that enable its colorchange capabilities. One such component is one or more base polymersthat form a conducting matrix. In some embodiments, the base polymercomprises one or more water-soluble, synthetic polymers. Examplewater-soluble, synthetic polymers include polyvinyl alcohol (PVA), poly(vinyl acetate), poly (vinyl alcohol co-vinyl acetate), polyvinylacetate-vinyl alcohol, poly (methyl methacrylate, poly (vinylalcohol-co-ethylene ethylene), poly (vinyl butyral-co-vinylalcohol-co-vinyl acetate), poly(vinyl alcohol)-acrylamide, polyvinylbutyral, polyvinyl chloride, poly(vinyl nitrate), substituted poly(vinylalcohol), carboxylated poly(vinyl alcohol), and poly(vinylchloride-co-vinyl acetate-co-vinyl alcohol), and mixtures thereof.

The base polymer is mixed with one or more acids to form a conductingelectrolytic composition. As described above, the one or more acids can,in some embodiments, be mixed with the base polymer to form a polymergel that is used to form the active layer. Example acids include glacialacetic acid (CH3CQOH), propionic acid (C3H6O2), hydrochloric acid (HCl),hydrofluoric acid (HF), phosphoric acid (H3PQ4), acetic acid(non-glacial) (CH3CQOH), sulfuric acid (H2SO4), formic acid (CH2O2),benzoic acid (C7H6O2), nitric acid (HNO3), phosphoric acid (H3PQ4),sulfuric acid (H2SO4), tungstosilicic acid hydrate(H4[Si(W3Q10)4]·xH2O), hydriodic acid (HI), carboxylic acids(CnH2n+1COOH), dicarboxylic acid (HO2C—R—CO2H), tricarboxylic acid(C6HsO6), oxalic acid (C2H2O4), hexacarboxylic acid (C12H6O12), citricacid (C6HsO7), tartaric acid (C4H6O6), and mixtures thereof.

The base polymer is also mixed with one or more oxidants to enable theactive layer to be placed in an oxidized state. Example oxidants includealuminum nitrate (Al(NQ3)3), ammonium dichromate ((NH4)2Cr2O7), ammoniumperdisulphate (APS) ((NH4)2S2Os), barium nitrate (Ba(NQ3)2), bismuthnitrate (Bi(NQ3)3·5H2O), calcium hypoperchlorate (Ca(CIO)2), copper (II)nitrate (Cu(NQ3)2), cupric nitrate (Cu(NQ3)2), ferric nitrate(Fe(NQ3)3), hydrogen peroxide (H₂O₂), lithium hydroxide monohydrate(LiOH), magnesium nitrate (Mg(NQ3)2), magnesium perchlorate (Mg(CIQ4)2),potassium chlorate (KClO3), potassium dichromate (K2Cr2O7), potassiumpermanganate (KMnO4), sodium hypochlorite (NaClO), sodium periodate(NaIO4), zinc nitrate hydrate (Zn(NQ3)2), ammonium nitrate ((NH4)(NQ3)),silver nitrate (AgNO3), benzoyl peroxide (C14H10O4), tetranitromethane(CN4Qs), sodium perchlorate (NaClO4), potassium perchlorate (KClO4),potassium persulfate (K2S2Os), sodium nitrate (NaNO3), potassiumchromate (K2CrO4), and mixtures thereof.

In addition to a base polymer, an acid, and an oxidant, the active layerfurther comprises one or more water-soluble dyes having a nitrogen groupthat produces a lone-pair effect (i.e., a condition in which pairs ofvalence electrons are present that are not shared with another atom).The dyes can be generally classified by the presence of a chromophoregroup. The N═N along with the bonding with one to two chromophore ringsare the azo dyes. Similarly, azo chromophore, ═C═O and ═C═C═ groupscontaining dyes, are called anthraquinone dyes. Because dyes areelectrochemically active and higher oxidized or reduced potentials cantransform the dyes, it is important to understand the reductionproperties of the dyes before their use in electrochemical processes. Insome embodiments, the dye or dyes used have a high reduction potential(i.e., the voltage at which a chemical species acquires electrons andthereby becomes reduced) such as greater than approximately −0.7 V, sothat the dyes retain their color and do not breakdown when relativelysmall negative potentials are applied. In some embodiments, the dye ordyes used also have an oxidation potential (i.e., the voltage at which achemical species loses electrons and thereby becomes oxidized) ofapproximately 2 V to −0.7 V, so as to be able to achieve good colorcontrast. Examples of such dyes include methyl orange (MO), methylviologen (MV), eosin Y (EO), conga red (CR), rhodamin B (RB), methyleneblue (MB), allura red (AR), crystal violet, acid fuschin, nigrosine,cationic dye, orange G, and mixtures thereof.

The active layer also comprises one or more conducting polymers having anitrogen group. In some embodiments, the conducting polymer or polymersused have an oxidation and reduction potential in the range ofapproximately 2 V to −0.7 V. Example conducting polymers includepolyanilines (e.g., polyaniline (PANT), poly(ortho-anisidine) (POAS),poly(o-toluidine) (POT), poly(ethoxy-aniline) (POEA)), substitutedpolyanilines, polypyrroles, substituted polypyrroles, polythiophenes,polycarbazole, substituted polycarbazole, polyaniline-rhodamine,rhodamine, polythiophene-rhodamine, and mixtures thereof.

Once a mixture of the various components identified above has been isformed, it can be deposited on a transparent or translucent layers usingany one of a variety of techniques, including electrochemically, bysolution cast, or using a self-assembly technique to form a singleactive layer. When a positive potential is applied to the active layer,for example using the electrically conductive inactive layers, the dyeand the conducting polymer within the active layer are each oxidized andeach adopts a relatively dark color. These colors combine to create arelatively dark “combination color,” i.e., a hybrid color that resultsfrom the combination of the color of the dye and the color of theconducting polymer. Because of the dark combination color produced bythe dye and the conducting polymer, the active layer can, at least insome embodiments, become opaque and, therefore, prevent light frompassing through the active layer. As such, the first colored state ofthe electrochromic device can be a light-blocking state in which theactive layer prevents light from passing through the device. When anegative (reversing) potential is later applied to the active layer suchthat the active layer has little or no electrical potential, the dye andthe conducting polymer are each reduced. As a result of this reduction,the dye adopts a relatively light color, while the conducting polymerbecomes completely or substantially transparent. As a result, thecombination color for the active layer in the reduced state is primarilythe light color of the dye in its reduced state. As such, theelectrochromic device is placed in a second colored state that can be alight-transmitting state in which the active layer enables light to passthrough the device. The particular colors and shades that result whenthe active layer is the oxidized (dark) and reduced (light) statesdepend upon the particular dyes and conducting polymers that are used,as well as the particular electrical potentials that are applied.Examples of active layers in the oxidized and reduced states areprovided below.

Examples of Embodiments

The general construction and operation of the disclosed electrochromicdevices having been described above, specific examples of electrochromicdevices will now be discussed and illustrated.

PVA+APS+dye+PANI active layers were fabricated and characterized usingscanning electron microscopy (SEM), ultraviolet-visible (UV-vis)spectroscopy, cyclic voltammetry (CV), and chronoamperometrictechniques. The coloration and decoloration of the active layer werestudied by measuring UV-vis absorption from 350 nm to 900 nm at 2 V and−0.7 V. The cyclic voltammetry at various scan rates was also studied tounderstand the reversibility process and diffusion-controlled processesin the various dyes. The color-change mechanism can be understood byanalyzing the redox states of dye and PANI in PVA+APS+dye+PANI activelayer based electrochromic devices.

Materials and Methods

Preparation of PVA Gel:

Initially, PVA gel was prepared by dissolving 50 gm of PVA in a solutionof 500 ml of 1 M HCl in a round bottom flask. The solution was heated to80° C. and maintained at that temperature for 12 hours. Later, thesolution was cooled at ambient temperature and permitted to gel forseveral days (more than a week before the use). In the discussions thatfollow, “PVA” is on occasion used to refer to this PVA gel, whichincludes HCl as an acid.

PVA+APS+PANI Gel

The PVA gel was used to prepare PVA+APS based gel electrolyte.Separately, M of APS solution was prepared in 1 M HCl. 10 ml of solution(0.1 M APS in 1 M HCl) was added with 40 ml of PVA gel and stirred foran hour. The reaction with APS in PVA created the oxidized PVA+APS gel.The 50 ml gel containing PVA+APS was added with 5 ml of aniline solutionwith an interval of 3 minutes. The aniline was oxidatively polymerizedin PVA gel in the presence of APS oxidizer. The obtained gel is referredto as PVA+APS+PANI.

PVA+APS+Dye+PANI Gel:

Initially, 0.01 M of each dye (MO, MV, EO, CR, RB, MB, and AR) wasdissolved in a solution containing 0.1 M APS and 1 M HCl. The resultingsolution of 10 ml was added to 3 ml at an interval with 40 ml preparedPVA+APS gel under continuous stirring conditions. Later, 5 ml of anilinewas added to the solution of PVA+APS+dye (MO, MV, EO, CR, RB, MB, andAR) and stirred for 12 hours at room temperature. An exception was madefor the AR dye, which was available as a semi-solid product. 5 ml of ARdye was added to 10 ml of the PVA+APS+dye+PANT gel, which is very stableand can be stored for months. Table 1 identifies each dye in thepreparation of PVA+APS+dye (MO, MV, EO, CR, RB, MB, or AR)+PAM basedactive electrochromic material. FIGS. 2A and 2B show the structures ofvarious dyes and oxidized and reduced states of dye containing activelayer based electrochromic devices (the images were obtained from videoin the oxidized state at a potential of 2 V and the reduced state at apotential of −0.7 V).

TABLE 1 Experimental conditions and gels used to obtain a single activelayer electrochromic material. Dye added in 40 ml 10 ml of of (0.1M PVAReaction of APS + 1M gel aniline with HCI with PVA + No. Active layergel (0.01M)) HCI Aniline APS + dye 1 PVA + APS + 0.1636 g 40 ml 5 ml 12hrs. MO + PANI 2 PVA + APS + 0.1285 g 40 ml 5 ml 12 hrs. MV + PANI 3PVA + APS + 0.3239 g 40 ml 5 ml 12 hrs. EO + PANI 4 PVA + APS + 0.3483 g40 ml 5 ml 12 hrs. CR + PANI 5 PVA + APS + 0.2395 g 40 ml 5 ml 12 hrs.RB + PANI 6 PVA + APS + 0.1599 g 40 ml 5 ml 12 hrs. MB + PANI 7 PVA +APS + Added 5 ml 40 ml 5 ml 12 hrs. AR + PANI (cone. sol)Results and DiscussionsSEM Studies:

FIG. 3 shows SEM pictures of PVA+APS+PANI and PVA+APS+dye (MO, MV, EO,CR, RB, MB or AR)+PANI as an active layer deposited between twoFTO-coated glass plates. The solution of each dye-containing activelayer material was spread on FTO-coated glass plate and left to dry for24 hours. Later, the samples were heated at 40-50° C. to remove anywater, which could have evaporated after normal drying at roomtemperature. FIG. 3A shows a PVA gel having a PANI structure. The filmshows both flat and rough surfaces arising due to the drying process.FIG. 3B shows axe-like structures in the dried the PVA+APS+MO+PANIlayer. However, more flake-like structures are visible throughout thesurface dried in the PVA+APS+MV+PANI film, as shown in FIG. 3C. FIG. 3Dshows a rougher and charged surface due to SEM beam potential certainlydue to less conducting surface. Nearly equal flakes are randomly packedin the gel containing the VA+APS+CR+PANI based single layer structure inFIG. 3E. The RB dye in the PVA+APS+RB+PANI layer shows mountain andvalley structures beside sharp needle structures throughout the surfacein FIG. 3F. FIG. 3G shows a layered and smooth structure in thePVA+APS+MB+PANI layer due to presence of MB dye. However, the AR dyelayer reveals uniformly distributed wires or needles throughout thestructure. The presence of dye changes the morphology of thePVA+APS+PANI based structure.

UV-Vis Studies:

Curve 1 in FIG. 3 shows UV-vis absorption of the PVA+APS+PANI basedactive layer film coated on a glass plate. The curve shows absorptionpeaks at 807, 498, 421, and 390 nm. The peaks at 421 and 498 nm are dueto polaron and bipolaron states, however, the doping shows a wide peakat 807 nm. The peak at 390 nm is due to n-n* transition. The peak at 498to 421 nm is due to the presence of MO dye due to the azobenzene groupand shifted from characteristics 465 nm. Curve 2 in FIG. 4 shows theabsorption peaks for the PVA+APS+MO+PANI layer at 538, 516, 496, 456,and 374 nm. The curve has characteristic MO peaks, MO being an anionicdye and the absorption peaks are shown at 538 and 516 nm. Curve 3 ofFIG. 4 shows absorption peaks positioned at 858, 829, 663, 575, 502, 428and 377 nm for the PVA+APS+MV+PANI layer. The MV dye had characteristicpeaks at 377 and 577 to 502 nm due to the cationic radical, which couldbe due to oxidizer. The curve shows characteristic absorption peaks ofMV dye at 663, 572 and 402 nm. The MV had a characteristic peak at 446,however, it was shifted 406 nm.

The UV-vis absorption peaks for the PVA+APS+ES+PANI layer are shown inCurve 4 of FIG. 4 . The curve reveals peaks at 828, 506, 482, 447, 405,381, and 364 nm. Curve 5 shows the UV-vis peaks at 828, 506, 482, 447,405, 381 and 364 nm due to the presence of CR in PVA+APS+CR+PANI. Thepeak observed at 482 nm is due to the CR red dye and blue shifted due tothe concentration of acid in the structure. The characteristic RB isshown in Curve 6 of FIG. 4 at 561, 530, 499, 468, 438, 387, and 360 nm,and characteristic MB and PANI peaks are shown in Curve 7 of FIG. 4 at758, 738, 676, 619, 498, 445, 404, and 362 nm. Regardless, the peaks ofdominant PANI and dye in each UV-vis absorption curve have theabsorption peaks at around 800 nm due to the presence of the dopant formof PANI.

FIG. 5A shows the UV-vis absorption of PVA+APS+MO+PANI at 1.7 V (Curve1) and −0.5 V (Curve 2). Interestingly, the absorption magnitude at 1.7V saturates the absorption from 600 to 870 nm, whereas thecharacteristic color (light red-pinkish color) is observed at −0.7 V. Apurple-pinkish color is also observed for the single layer electrolytefor the potential at −0.5 V for PVA+APS+CR+PANI layer in FIG. 5B (Curve1).

The presence of CR dye provides the dark color, and the color contrastis different than with the presence of MO dye in the single layer gel inFIG. 5B (Curve 3). FIG. 5C shows variation of dark to red color when thepotential is applied at −0.5 V to 1.7 V. The sharp characteristic peakat 561 nm peak is present at the oxidized potential (Curve 1) and thereduced potential at −0.5 V (Curve 2) in FIG. 5C.

Cyclic Voltammetry Studies:

CV studies on PVA+APS+dye (MO, MV, EO, CR, RB, MB or AR)+PANI activelayer sandwiched between two conducting FTO-coated glass plates werealso studied. FIG. 6A shows the CV studies of the PVA+APS+MO+PANI layerat various scan rates. The CV shows the complete CV curve and it isreversible, however, there is a change in the CV pattern at differentscan rates due to changes in the diffusion properties of the activelayer. However, it shows the oxidation peak at 1.55 V and reductionpeaks at 0.81 V and −0.58 V. The oxidation peak is related to theoxidized pernigraniline state as well as the oxidized state of the MOdye dark color, whereas the two reduction peaks are due to theemeraldine and leucoemeraldine forms of PAM. The pinkish color isreduced for the MO dye, which has appeared in at potential −0.7 V. TheCVs of the PVA+APS+MV+PANI active layer shows an oxidation peak at 1.55V and two reduction peaks 0.95 V and −0.59 V. The PANI is reduced toleucoemeraldine. The CVs of PVA+APS+EO+PANI active layer shows oxidationpeak at 1.54 V and reduction peaks at 0.92 and −0.563 V (FIG. 6C)similar to the PVA+APS+MV+PANI active layer in FIG. 6B. The MV undergoesthe two reduction states between −0.4 to −1.1 V, where we have onlyscanned till −0.7 V. So, only one reduction state can be observed.However, the presence of CR dye in the PVA+APS+CR+PANT active layershows a pinkish color at reduced states. It has the oxidation peak at1.657 V and reduction peaks at 0.45 V and −0.63 V (FIG. 6D). Aninteresting color feature has been observed for RB dye in thePVA+APS+RB+PANI active layer. It has oxidation peak at 1.62 V and thereduction peak at 0.993 V, as shown in FIG. 6E. However, the completeleucomeraldine peak is missing due to the presence of RB dye, which hasa higher native reduction potential, and the active layer color isdominated by the presence of RB dye. The presence of MB dye shows asimilar effect as observed previously. It has oxidation peaks at 1.32 Vand 0.8 V and the reduction peaks are at 0.89, 0.29, and −0.58 V. Thecomplete leucoemeraldine state can be achieved in presence of MB dye inPVA+APS+MB+PANI as active layer, as shown in FIG. 6E.

FIG. 7 shows the chronoamperometric results of PVA+APS+PANI andPVA+APS+dye (MO, MV, EO, CR, RB, MB)+PANI active layer deposited onFTO-coated glass plates. The oxidation and reduction process isasymmetric in each electrochromic device. There is a marked differencebetween the oxidation and reduction processes in the active layercontaining CR and RB dye than the rest of studied dyes. The reduction issharper in each studied active layer. However, oxidation shows twooxidizing phases in each studied dye-based active layer.

Color-Change Mechanism

An attempt was made to understand the mechanism of coloration anddecoloration using various devices containing a dye-based active layerbetween two FTO-coated glass plates. The oxidation of each dye resultsin a dark color regardless of the nature of dye, whereas PAM changes topernigraniline in the potential range of 1.5 to 1.8 V and dye is alsooxidized to darker color. The combination colors are even darker in theoxidized states. However, the two switching color states can be observedwhen the reduction of the dye is at a potential higher than −0.7 V orthe colored reduction state is present at a potential of −0.7 V.

FIG. 8 shows the oxidation and reduction states of a PVA+APS+MO+PANIbased active layer. It is interesting to note that the oxidation stateis related to an oxidized state of nitrogen in the MO dye as well as inthe PAM structure. The reduction brings to original state of MO dye forthe reduction potential is between −0.7 V. So, a purple-pinkish color todark color is observed in the MO-based electrochromic device. FIG. 9shows the oxidation of CR dye at a nitrogen bond in the dye structure.The oxidation and reduction are similar to MO-based active layers. Thereduction shows the purple color, which is the reduction state.

The most interesting color contrast between two color states wasobserved for RB dye containing active layers. The nitrogen grouppartially oxidizes in the RB dye and the reduction shows the return of ared state of the RB dye in the active layer (see FIG. 10 ). However, theoxidation and reduction of PANI occurs in the potential range from 2.0 Vto −0.7 V. RB dye is dominant in the reduction potential and a red todark color has been observed.

In conclusion, color change between two colored states (e.g., red toblack, red to blue, purple to black, etc.) of an electrochromic devicehas been described. A PVA gel containing PANI and APS was tested withvarious dyes. It is important to understand the electrochemistry of eachdye with the PVA+APS+PANI system. The oxidation and reductioncharacteristics of each dye is important and the oxidation potential ofthe dye should be below 2.0 V whereas the reduction potential of the dyeshould be greater than −0.7 V. The reduction potential of the dye shouldbe high to retain its color, otherwise only dark to transparent stateswill be possible. The mechanism of multicolored states has beenunderstood using chemical structures.

The invention claimed is:
 1. An electrochromic device comprising: asingle unitary active layer containing: a dye having a nitrogen group, aconducting polymer having a nitrogen group, said conducting polymerbeing polyaniline, and polyvinyl alcohol (PVA), hydrochloric acid (HCl),and ammonium perdisulphate (APS) ((NH₄)2S₂O₈); wherein the singleunitary active layer is configured to assume a first color in a firstoxidized state, a second color in a reduced state, and a third color ina third state between the first oxidized state and the second reducedstate; wherein said dye having a nitrogen group comprises one or more ofmethyl orange (MO), methyl viologen (MV), eosin Y (EO), congo red (CR),rhodamin B (RB), methylene blue (MB), allura red (AR), crystal violet,acid fuschin, nigrosine, cationic dye, and orange G; and wherein theelectrochromic device comprises no other active layer.
 2. Theelectrochromic device according to claim 1, further comprising first andsecond transparent or translucent layers of material between which thesingle unitary active layer is positioned.
 3. The electrochromic deviceaccording to claim 2, wherein the single active unitary layer isconfigured to be substantially light-blocking in the first oxidizedstate to prevent light from passing through the device.
 4. Theelectrochromic device according to claim 2, wherein the first and secondtransparent or translucent layers comprise thin transparent glass orplastic plates that are coated with a transparent electricallyconductive film.
 5. The electrochromic device according to claim 1,further comprising a voltage source configured to apply an electricalpotential to the single unitary active layer.
 6. The electrochromicdevice according to claim 1, wherein the single unitary active layer isconfigured to be placed in the first oxidized state when a positiveelectrical potential is applied to the single unitary active layer andis configured to be placed in the second reduced state when the positiveelectrical potential is reversed by applying a negative electricalpotential to the single unitary active layer, and wherein the singleunitary active layer is configured to be reversibly transitioned fromthe first oxidized state through the third state to the second reducedstate by controlling an electrical potential applied across the activelayer.
 7. The electrochromic device according to claim 6, wherein thefirst color is a relatively dark combination color that results fromoxidation of both the dye and the conducting polymer and the secondcolor is a relatively light combination color that results fromreduction of both the dye and the conducting polymer.
 8. Theelectrochromic device according to claim 6, configured to have positiveand negative values of said positive and negative electrical potentialapplied to the single unitary active layer range from approximately 2 Vto −0.7 V.
 9. An electrochromic device comprising: a single unitaryactive layer that has a first color in an oxidized state, a second colorin a reduced state, and multiple colors corresponding to multiple statesthat are between the oxidized state and the reduced state, the singleunitary active layer including: a base polymer that is polyvinyl alcohol(PVA); an acid that is hydrochloric acid (HCl); an oxidant that isammonium perdisulphate (APS) ((NH4)₂S₂O₈); a dye having a nitrogengroup, and a conducting polymer having a nitrogen group, said polymerbeing polyaniline (PANI); and first and second transparent ortranslucent layers of material between which the single unitary activelayer is disposed; wherein the device comprises no other materiallayers.
 10. The electrochromic device according to claim 9, wherein thesingle unitary active layer is configured to be placed in the oxidizedstate when a positive electrical potential is applied to the activelayer, and to be placed in the reduced state when the positiveelectrical potential is reversed by applying a negative electricalpotential to the single unitary active layer; and wherein the singleunitary active layer is configured to be reversibly transitioned fromthe oxidized state through the multiple states to the reduced state bycontrolling an electrical potential applied across the active layer. 11.The electrochromic device of claim 10, wherein the first color is arelatively dark combination color that results from oxidation of boththe dye and the conducting polymer and the second color is a relativelylight combination color that results from reduction of both the dye andthe conducting polymer.
 12. The electrochromic device of claim 11,configured to have positive and negative values of said electricalpotential applied to the single unitary active layer range fromapproximately 2 V to −0.7 V.
 13. The electrochromic device according toclaim 9, wherein said dye having a nitrogen group comprises one or moreof methyl orange (MO), methyl viologen (MV), eosin Y (EO), congo red(CR), rhodamin B (RB), methylene blue (MB), allura red (AR), crystalviolet, acid fuschin, nigrosine, cationic dye, and orange G.
 14. Theelectrochromic device according to claim 9, wherein in the single activeunitary layer is configured to be substantially light-blocking in thefirst oxidized state to prevent light from passing through the device.